Commutation signaling is known for example from an article by George L. Turin entitled "Commutation Signaling--An Antimultipath Technique", IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 4, pages 548-562, July 1984. Commutation signaling using differential encoding is particularly advantageous for high speed digital transmission over multipath fading channels. Information to be transmitted is differentially encoded, for example using DPSK (differential phase shift keying), and multiplied by sequential ones of a set of substantially orthogonal codewords; thus commutation signaling is a spectrum spreading technique. An M.times.N commutation signaling scheme uses N codewords each of M bits. A receiver structure for use in a communications system using such commutation signaling is shown in FIG. 8(c) of that article, and forms the basis for the prior art in FIG. 1 described below.
An article by Harry Leib entitled "A Digital Transmission Approach For Indoor Millimeter Waveband Systems", Proceedings of the 2nd International Conference on Universal Personal Communications, Ottawa, Ontario, Canada, pages 880-884, Oct. 12-15, 1993, suggests the application of SAW devices to a receiver, including matched filters, in such a system, but does not provide details.
An article by H. Nakase et at. entitled "One Chip Demodulator Using RF Front-End SAW Correlator For 2.4 GHz Asynchronous Spread Spectrum Modem", in the Proceedings of the 5th IEEE International Symposium on Personal, Indoor, Mobile Radio Communications, The Hague, The Netherlands, pages 374-378, September 1994 describes a SAW correlator in which a signal spread by a PN (pseudo-noise) code is propagated as a SAW to a tapped delay line with taps matched in phase to the PN code. This article also illustrates a SAW delay line used for differential decoding. This article does not relate to commutation signaling.
Similarly, Shinonaga U.S. Pat. No. 5,056,055 issued Oct. 8, 1991 and entitled "Coherent Surface Acoustic Wave Unique Word Detector" discloses a SAW matched filter having an output IDT (inter-digital transducer) which is coded, i.e. taps along the length of which are matched in phase, in accordance with the bits of the unique word to be detected.
An object of this invention is to provide a practical and advantageous surface wave device for use in differential coherent detectors of communications systems such as those using commutation signaling.